The present invention relates to solar-powered devices and in particular to timepieces of the type disclosed in Yamada, "`No Battery` Analog Quartz Watch," Proceedings No. 2 of the XI Internat Conf. for Chronometry, Paper #VA1, pp. 75-79 (Besancon, October 1984). In general, such a timepiece includes an array of solar cells to convert ambient illumination into electric current that drives a load consisting of the timepiece movement and whatever other function modules may be included. To provide continuous operation of the timepiece through periods of low-level light due to night or obscuration by clothing, an energy storage device such as a capacitor is charged by the solar cells when the light on the solar array is bright enough and discharged into the load when the light dims. The typical user certainly prefers the timepiece to function immediately after its initial exposure to light and to become fully charged as quickly as possible. Unfortunately, a quick-responding capacitor cannot store enough energy to drive the load for a usefully long time while a capacitor large enough to drive the load long enough cannot quickly reach an adequate supply voltage level.
As disclosed in FIG. 13 of the paper cited above, in addition to the charge and discharge circuits for a very high capacity capacitor serving as the operating storage, a small auxiliary capacitor is provided, the latter being connected in parallel directly to the watch circuit and through a return blocking diode to the solar cell. It is intended by this layout that in view of the small time constant of the lower value capacitor, the timepiece will start after only a brief illumination of the solar cell.
The disadvantage of this use of auxiliary starting means consists of the price and the space requirements of two capacitors and from the technical operating standpoint in particular the low capacity of the starting storage means. It results from this low capacity that while the watch is actuated even after only a brief exposure to light, the operation of the watch soon comes to a halt if the illumination was excessively brief. Thus, if a user ascertains, for example by actuating the illumination of a room, that the solar watch has started, he cannot be sure that it will run, for example through the night, if the room lights are turned off.
The use of rechargeable electrochemical charge storage means (batteries or secondary elements) is not feasible in the case of solar watches of this type, because the limited life of such elements, in spite of the buffering from the solar cell, requires the replacement of batteries from time to time by the user, which is exactly what the so-called battery-less solar watches of this type are intended to eliminate.
It has been found that to assure the continuous operation of a solar watch equipped with capacitive charge storage means the simple solution of significantly increasing the storage capacity is not applicable without further complications. Such an action would increase the charging time constant, whereby the rise of the voltage to the value required for the operation or in particular for the startup of the electric and electromechanical circuit components that consume the charge stored would be slowed to an unacceptable degree. The user cannot be expected to be willing to wait after the removal of the "no-battery" device from its packing for an extended period of time until the attainment of a regular function becomes recognizable.
Paradoxically, it is a fact that an increase in the capacity of the charge storage means may also entirely block the regular startup of a small device, for example, if the device contains circuit components characterized by high power requirements and/or clearly different minimum operating voltages. Due to the slow voltage rise forced by the high capacity, different parts of for example a watch circuit are actuated at very different points in time following the onset of the irradiation of the solar cell. The actuations of the parts may possibly again reduce the stored charge level in view of their high power requirements, so that the normal cooperation of the circuit components required for stable operation cannot be established. The device may even pass into a permanent relaxation oscillation state without ever attaining a stable operating condition.
Such unfavorable charging states immediately following the actuation of a device occur particularly when a radio watch is involved wherein at the onset of the operation more power is needed for operation of the radio receiver (and its decoding circuit for the time information received) and for the accelerated movement of the gears during the motion of the hands into their reference positions. Similarly unfavorable operating conditions, in particular immediately following the actuation of the device, are present if, in the course of activation certain decorative elements, such as ornamental pendulums, jerking figures or the like, must be set into motion. This is particularly the case when an electromechanical converter such as a stepper motor is provided, the high starting current of which is reduced to a stable operating level only after the onset of for example rotor movement by means of induction effects. In such cases, the high initial power needs may lead to a condition wherein the charging state of the capacitor cannot attain the voltage value required for the continuous normal operation of the entirety of the circuit components so that unrestricted actuation from an uncharged capacitor state is not assured.